1. Field of the Invention
A slowly rotating synchronous generator for a wind power installation and, more particularly, a ring generator having a circuit for inducing a current in a stator of the generator.
2. Description of the Related Art
Slowly rotating electrical machines, for example ring generators, as are used in the wind power installations from Enercon of types E-33, E40, E-12 and E-66, require very high excitation power. The excitation power required rises with the number of poles, with a rising air gap, and with the level of the reactive power.
Ring generators of the above-indicated kind have, for example, 72 or 84 poles. The efficiency of directly driven generators for use in the area of wind power should be as high as possible because they are in operation as far as possible for 24 hours a day.
Slowly operating ring generators for wind power installations, such as, for example, those of type E-66 from Enercon, operate at the rotary speed range of between 10 and 22 rpm. Such a ring generator is constructed for example with 72 poles (36 pairs of poles) and thus produces a frequency of between 6 and 13.2 Hz.
Such a ring generator comprises a rotor, through the windings of which the excitation power is built up, and a stator that surrounds the rotor. Compensation in respect of the reactive power or overcompensation of the stator with capacitors is very expensive because, as described above, the frequency is very low.
The capacitor current is generally calculated in accordance with the formula:       i    c    =      C    ·                  ⅆ        u                    ⅆ        t            
In that respect, for sinusoidal voltages (as in the case of known generators), there is a capacitor current ofic=U·2·π·f·C
The capacitor current is therefore determined by the voltage, the capacitance of the capacitors, and the applied frequency.
With a generator frequency of, for example, between 6 and 13.2 Hz, there is unfortunately only a small capacitor current in comparison with a conventional frequency of 50 or 60 Hz. The sinusoidal capacitor current admittedly involves a phase shift of 90° relative to the active current, but it flows in each case over a range of 180° and in that situation causes increased copper losses in the stator winding.
German Patent DE 42 18 298 discloses a permanently excited generator system, wherein a synchronous generator has a rotating magnetic field that is regulatable by way of a voltage detector for detection of the output voltage of the permanently excited synchronous generator and a comparator for comparing the detected voltage by means of the voltage detector to a reference voltage which can be set by means of a voltage setting device.
U.S. Pat. No. 5,773,964 discloses a regulating system for an automobile generator.